The present invention relates to nylon resin blends, single and multilayer films and casings containing nylon resin blends for use in packaging. In particular, the invention relates to nylon blends, films and casings and/or bags made thereof which are suitable for packaging food products such as fresh meat, processed meat, cheese and sausages.
Nylon is the generic name for a family of polyamide polymers characterized by the presence of the amide group-CONH. The utility of nylon compositions and products are well known with everyday examples including usage in packaging, brushes, and tires, as synthetic films, fibers, plastics and molding resins.
Thermoplastic flexible films are used in a wide variety of applications including bags (e.g. for merchandise, leaves, garbage) wrappings for industrial packaging, in electrical and electronic uses, as communications media substrates, adhesive coated products such as tapes and labels, medical packaging, and food packaging.
In the food industry, thermoplastic flexible films are used to keep food fresh prior to consumption. Greater use of centralized processing of foods in conjunction with increased handling and longer delivery times associated with long distance transportation have increased the demand for packaging films having superior properties.
In the poultry and meat segments of the food industry thermoplastic flexible films are utilized to maintain freshness. Meat is frequently sold fresh, frozen or cooked; therefore films advantageously provide protection at various temperatures. Food items such as primal and subprimal cuts of beef, ground beef and processed meats are known to use coextruded or laminated films which utilize such compositions as nylon, polyester, copolymer of vinylidene chloride (PVDC), ethylene-vinyl acetate copolymer (EVA) and ionomers.
It is also generally known that selection of films for packaging food products includes consideration of such criteria as barrier properties, cost, durability, puncture resistance, flex-crack resistance, FDA approval, machinability, optical properties such as gloss and haze, printability, sealability, shrinkability, shrink force, stiffness, and strength.
In general, nylon films are made by processes which include casting or blown film and these films may be uni- or biaxially oriented. Specific types of nylon such as nylon 6, nylon 6,6, and nylon 12 have been made into films. Known advantages of nylon films relative to other film materials in packaging applications include good oxygen and flavor barrier characteristics, durability at low temperatures and thermal stability. However, nylons in general are costly and are poor moisture barriers. It is known to use certain nylon films as core layers in oriented multilayer films. However, nylon selection is critical and processing is very difficult. These multilayer films may include one or more additional layers of films made of various resins, for example, low density polyethylene (LDPE), ethylene-vinyl acetate copolymer (EVA), ionomer, PVDC, or copolymers of ethylene and methacrylate. Nylon containing films have also been used in vacuum packaging of fresh meat. Typical and generally known films suitable for packaging and information on film manufacture are described in the Encyclopedia of Polymer Science and Engineering 2nd Ed., Vol. 7, pp. 73-127, Vol. 10, pp 684-695 (John Wiley & Sons, Inc., 1987) whose teachings are hereby incorporated by reference.
Also, amorphous nylons have been disclosed as useful in thermoplastic films including multilayer films and biaxially drawn films.
U.S. Pat. No. 4,698,195 and European patent application No. 240,632 both disclose a process for preparation of a biaxially drawn polyamide film which utilizes what is termed "a substantially amorphous unoriented polyamide film".
Also, U.S. Pat. No. 4,668,571 (Moriarty, Jr.) discloses a multilayer stock material useful for producing flexible, thermoplastic bags in which an outer layer may comprise a polyamide resin. Among the examples of suitable polyamide resins is listed amorphous nylon.
Additionally, European patent application No. 236,099 (Fant et al) discloses a multilayer thermoplastic film having optional polyamide layers. Suitable polyamides are said to include a commercially available copolymer of nylon 6 and nylon 12 having a composition of about 60% nylon 6 and about 40% nylon 12 by weight sold under the trademark Grilon CA-6 by Emser Industries. Another suitable nylon copolymer is disclosed as CR-9, having 20% to 30% nylon 6 and 70% to 80% nylon 12 by weight.
Also, more recently coextruded film packaging for processed meat was reportedly being used in Japan which utilizes an amorphous nylon sold under the brand name Novamid X21 by Mitsubishi Chemical Industries Ltd. of Tokyo (See "Coextrusion Developments Focus on Barrier Resins", Plastics Technology, Vol. 33, No. 13, pp. 5, 77-79, December, 1987 Bill Communications, Inc., New York).
Oriented nylon films are also well known in the packaging industry for their toughness, puncture resistance, and oxygen barrier properties. In particular, biaxial orientation is known to generally improve film strength. The oxygen barrier properties of oriented nylon films generally provide greater resistance to oxygen permeability as the level of absorbed moisture decreases. As the moisture content increases, the oxygen barrier properties of most oriented nylons deteriorate. When nylon films are to be used or stored under humid or other moist conditions, it becomes desirable to protect the nylon film e.g. by placement between layers having relatively low permeability to moisture, in order to keep the nylon dry. However, orientation of coextruded multilayer blown films having nylon as a protected core layer is difficult due to processing constraints. Often nylon is the outer layer in coextruded processing because of the necessity for rapid quenching to achieve suitable processing in a multilayer structure. Of course, lamination processes may be and are presently utilized to attach a moisture protective layer to nylon, but this is disadvantageously expensive.
In many packaging applications, it is also desirable that at least one of the layers have good heat seal properties. Resins which have both good heat sealability and are substantially impermeable to moisture include various polyethylenes, ethylene copolymers and ionomers. Oriented nylon films are currently used alone and in combination with these heat sealable and moisture resistant layers.
Disadvantageously, it has proved difficult to find film layers (other than PVDC) which have good barrier properties to both moisture vapor and oxygen. Therefore, multilayer films are commonly employed to utilize the most beneficial properties of various film layers.
The Encyclopedia of Polymer Science and Engineering, 2nd Edition., Vol. 7, pp. 77-79 (John Wiley & Sons, Inc., 1987) discloses that "nylon is frequently the core portion of the film being coextruded or coated with sealant resins, such as LDPE, EVA, ionomers, or copolymers of ethylene and methacrylate". The nylon layer acts as an oxygen and flavor barrier for such applications as processed meat and cheese packaging, boil-in-bags, and bags for baked goods.
In a typical known process for producing multilayer films containing oriented nylon, the nylon film is oriented by heating to a softened state below the melting point and stretching the softened material. Many conventional nylon resins crystallize very rapidly and have melting points well in excess of adjacent polyethylene layers. Due to these temperature differences and because nylon and polyethylene tend to have different stretching characteristics, the nylon layer is typically oriented separately and in advance of its combination with the adjacent polyethylene layers. The combination of the oriented nylon with the adjacent layers is then accomplished using a conventional but relatively expensive and complex lamination process. This requires an adhesive such as polyurethane type adhesive applied with a coater-laminator.
Another problem with current multilayer oriented nylon structures is that, while a material such as polyethylene generally protects the nylon from moisture, some moisture gradually seeps in from either the packaged food article or the atmosphere and is absorbed by the nylon. This causes an increase in oxygen permeability which shortens the shelf life of oxygen sensitive foods.
Due to recent growth in the market for barrier films, there currently exists an industry wide search for films with improved barrier properties such as low oxygen permeability and low water permeability. For economic reasons, there is also a demand for an oriented nylon multilayer film which can be produced by a coextrusion process. Production of multilayer films by coextrusion is generally more economical than use of lamination methods.
The present invention provides an improved nylon resin blend, and single and multilayer films thereof which ameliorate many problems associated with known films.
It is not necessary that each and every problem listed above be overcome by all embodiments of the invention. It is sufficient that the invention may be advantageously employed when compared to the prior art.